The invention relates to the manufacture of sheet or flat glass by means of the float glass process, and more particularly to the delivery of homogeneous glass in molten sheet form to the molten metal bath of the float process.
As shown by U.S. Pat. Nos. 3,083,551 and 3,884,665, the float process is typically characterized by the delivery of a sheet of molten glass onto the surface of a bath of molten metal such as tin, wherein the glass is normally delivered from the broad surface of a continuous glass tank over a refractory lip and onto the central one third or one half of the width of the molten bath. The ribbon of molten glass flows outwardly upon the molten bath until the force tending to cause the spreading, represented by the thickness and density of the glass, and the force resisting the spreading, represented by the surface tension and the radius of curvature of the glass edge, have reached an equilibrium. However, the theoretical thickness of the glass at such equilibrium is about 0.28", which is much thicker than required for normal architectural window glass and automobile windows. Accordingly, in order to produce the required thinner sheet, edge portions of the expanded sheet flowing outwardly on the molten bath are engaged by knurled rollers and the velocity of the sheet downstream from such rollers is increased to draw down and produce thinner sheet as shown by U.S. Pat. No. 3,853,523.
However, unless the glass which is supplied to the molten bath has been well stirred so as to be completely homogeneous and free of cord, surface streak or ridges may appear in the drawn glass as a result of the attenuation of nonhomogeneous glass containing cord or striae. That is, since surface streak or ridges are produced by the attenuation of discontinuities or glassy inclusions possessing optical and other properties different from those of the surrounding glass, it is imperative that the sheet of molten glass delivered to the surface of the tin bath be well stirred and homogeneous so as to avoid the formation of surface streak during the drawing down and attenuation of the sheet or ribbon delivered to the surface of the molten metal bath. In view of the fact that the surface defect becomes more pronounced as the glass on the molten bath is drawn thinner, homogeneity of the supply glass is even more important when producing thin glass sheets.
Various methods of delivering a sheet of molten glass to the molten metal bath have been suggested in addition to the conventional over-the-lip delivery as disclosed in the aforesaid U.S. Pat. Nos. 3,083,551 and 3,884,665, such as those shown by U.S. Pat. No. 3,679,389. All of such deliveries, however, include extended longitudinal flows having a width equal to the width of the delivered sheet, and accordingly such deliveries necessitate massive free surface areas. However, due to surface volatilization and temperature differentials between the surface and the depths therebelow, composition discontinuities and nonhomogeneous glass result which detrimentally affect the optical quality of the sheet glass formed therefrom due to the inclusion of cord and striae.
It thus has been an object of the present invention to improve upon the known technology of forming float glass by delivering well stirred homogeneous molten glass in sheet form to the molten metal bath of the float glass process to provide improved optical quality in the flat glass formed thereby.